The rapid development of enthusiasm for optical communications evident over the past few years is attributable to a breakthrough in the quality of glass fiber lightguides for optical transmission. Optical fiber transmission lines can now be made with extremely low loss and acceptably low mode dispersion. With appropriate coating and cabling techniques the fibers can be handled and installed. However, we have found that fibers installed in long lines (ca. 100m) have erratic tensile properties and long fiber lengths often break unexpectedly. Analysis of the tensile properties of various fibers shows an unexpected variation in tensile strength with length pointing to uncommon structural failures occurring randomly along the fiber length. Further inquiry has shown that these failures occur from crack propagation at microcracks that occur unpredictably during the fiber drawing operation. Having established that the microcracks cause the fiber breaks, we then devised a quality control system to monitor the fiber as it is drawn or processed, to reveal the presence of these defects.
The monitoring technique is based on scattering of laser light from the fiber defects. Surface defects as well as included defects are within the term "defect".
Optical scattering of laser light has been proposed as a tool for analyzing the dimensional properties of optical fibers. Marcuse and Presby, The Bell System Technical Journal, Vol. 54, No. 1, pp. 3-15, January, 1975, describe the use of light back scattered from a fiber to reveal distortions in the interface between the core and cladding of so-called "clad" fibers. Presby, J. Opt. Soc. Amer., Vol. 64, No. 3, pp. 280-284 and Watkins, J. Opt. Soc. Amer., Vol. 64, No. 3, pp. 767-772, both describe the use of scattered light for measuring fiber dimensions and refractive indices. These three techniques rely on a single optical principle and that involves the analysis of light reflected from or refracted through a curved transparent surface. The light pattern that gives the relevant data in each case is that appearing in the plane containing the light beam and normal to the axis of fiber.